It is known to selectively enable or disable a vehicle air bag or other occupant protection device based on the presence of an occupant in a seat. It has been proposed to place electrically conductive material in a vehicle seat to serve as an electrode for detecting the presence of an occupant in the seat. For example, U.S. Patent Application Publication No. 2009/0267622 A1, which is hereby incorporated herein by reference, describes an occupant detector for a vehicle seat assembly that includes an occupant sensing circuit that measures the impedance of an electric field generated by applying an electric signal to the electrode in the seat. The presence of an occupant affects the electric field impedance about the electrode that is measured by the occupant sensing circuit. However, environmental conditions such as humidity or moisture may interfere with the accuracy of measuring the electric field impedance. Furthermore, such measurements may become unreliable or unusable if liquid is present on or in the seat such as due to a wet bathing suit or due a window being left open during a rain shower. What is needed is a system that can determine the presence of an occupant in a seat having an electrode that is not adversely or unacceptably sensitive to varying humidity levels and can sense when a seat is wet.